In a wafer inspection apparatus for obtaining an image of a portion of a wafer and inspecting the image, in general, lighting having a single wavelength pulse is radiated to a corresponding area of the wafer while the wafer passes through a predetermined area for photographing (capturing image). Furthermore, a field of view (FOV) in which a lens unit may obtain an image is illuminated through a single piece of pulse lighting. Reflected light from the field of view passes through the lens unit, and an image of the photographing area (object area) of the wafer is focused on the pixel unit of an image pick-up element which functions as a screen at the back of the lens unit. After the photographing area of the wafer is photographed, in a next pulse lighting time, the wafer is moved so that a next photographing area adjacent to the photographed area is photographed.
In order to photograph all the areas of a wafer, assuming that a pulse lighting time is very short and the wafer is rarely moved during the pulse lighting time, the wafer has to move in the width direction during a pulse lighting cycle by the width of a field of view to be photographed, which may be once photographed by the image pick-up element.
However, to photograph a field of view to be photographed which has been radiated by lighting using a single image pick-up element requires a very long time taken to inspect the entire wafer because an existing image pick-up element has a limited capacity. Although a high-capacity image pick-up element is used, it is not appropriate because a lot of time is taken by a computer system connected to the image pick-up element and analyzing an image.
Accordingly, there is used an area sensor type wafer image inspection apparatus in which a plurality of unit image pick-up elements is disposed in the entire photographing unit so that they form a focal plane array (FPA) so as to increase a wafer area which may be photographed once and in which the image pick-up elements are analyzed using a single computer so as to reduce the time taken to inspect the wafer.
In the focal plane array, however, it is practically difficult to dispose a plurality of unit image pick-up elements closely. Row and column lead wires for drawing information signals, corresponding to images focused on pixel areas, to the outside need to be installed in each of the image pick-up elements in addition to the pixel areas to which the images are inputted. In order to install such lead wires, an installation area or an installation space around the pixel area is inevitable. If we take such a lead wire installation space into consideration, it is hard to think that the pixel areas of the plurality of image pick-up elements are closely disposed in a matrix form.
Accordingly, there is used a method for spatially separating and installing a plurality of unit image pick-up elements to be included in the virtual matrix of a unit image pick-up element which will be disposed in a focal plane on which an image of a photographing area of a wafer is focused, dividing the image to be focused on the focal plane for each area using an optical element, and distributing the split images to the respective image pick-up elements that are spatially separated and installed.
A wafer inspection apparatus for photographing the entire valid area of a wafer using spatially split image pick-up elements, analyzing a corresponding image, and detecting a defect using such a method is disclosed in Korean Patent No. 1113602 by Negevtech Ltd. A perspective view of FIG. 1 shows the concept of such a conventional wafer image inspection apparatus.
In such an apparatus, an image of a focal plane is split using a plurality of unit image pick-up elements forming the focal plane, that is, two-dimensional detectors 87a, 87c, 87d, 87e, and 87f and at least one optical element functioning to divide the image of the focal plane between the two-dimensional detectors using a beam splitter 69 of a glass plate form, prisms 89a, 89b, and 95, and a mirror.
In such an apparatus, in order to secure an accurate image of a field of view, the plurality of unit image pick-up elements is always disposed to form a focal plane array (FPA). An image other than the focal plane array is always the subject of resetting of equipment.
Meanwhile, in a semiconductor apparatus, a method for forming a circuit device by integrating circuit elements, such as devices and conducting wires, on a plane in a small size and continuing to reduce the size of the devices and wires so as to improve the degree of integration was used. However, as the degree of integration of devices is increased, it has become difficult to reduce the size of the devices and wires due to several limits in terms of a process for fabricating the semiconductor apparatus, for example, the optical limit of a photolithography process. Furthermore, to reduce the size of the devices has reached the state in which a device function may be problematic.
In such a condition, in order to improve the degree of integration of the devices of a semiconductor apparatus, a three-dimensional apparatus configuration, such as the multi-layering of the semiconductor apparatus and the three-dimension of the device configuration, is chiefly being attempted.
After a semiconductor apparatus is fabricated through a highly precise and complicated multi-stage process, an inspection task for checking whether the semiconductor apparatus has been normally formed as designed and whether the semiconductor apparatus may perform its unique function plays a very important role in discovering a process failure and finding out and correcting a problem so as to improve efficiency and effectiveness of the process.
Inspection equipment that belongs to pieces of existing semiconductor apparatus inspection equipment and that uses an image obtains an image of part of a target semiconductor apparatus and checks whether a semiconductor apparatus is defective by determining whether the image is normal. The three-dimensional configuration of semiconductor apparatuses has a problem in that an inspection cannot be performed sufficiently and properly using a conventional plane inspection method for the semiconductor apparatus.
If patterns are too small, it is difficult for a lighting beam to reach the patterns through the patterns. An optical microscope produces meaningful resolution results only if it is greater than half the size of a wavelength of light used. In the inspection of small patterns, such as the inspection of a semiconductor apparatus, a microscope user may uses a method for grouping and arranging similar patterns at a constant interval, observing how light is distributed between the groups, and determining the size. Such a method has many difficulties in measuring a new three-dimensional structure of semiconductor apparatuses.
A non-optical measuring method may be taken into consideration. It is difficult to use a non-optical image processing method, such as a scanning probe microscopy, as a practical inspection apparatus because the scanning probe microscopy is expensive and slow.
Recently, Ravikiran Attota, etc. who work for the National Institute of Standards and Technology (NIST) proposes a possibility that a three-dimensional fine pattern may be measured using a through focus scanning optical microscopy (TSOM). (“TSOM method for semiconductor metrology”, Proc. SPIE 7971, Metrology, Inspection, and Process Control for Microlithography XXV, 79710T, Apr. 20, 2011).
In this technology, an existing optical microscope is used, but a method for collecting two-dimensional images of the same object at different focal positions and producing a three-dimensional image data space of the object is used. Accordingly, the obtained two-dimensional images form a through-focus image, including an in-focus image and some out-of-focus images. A computer performs processing on such a three-dimensional image data space. The computer extracts a brightness profile from a plurality of collected through-focus images of the same object and generates an image of a through-focus scan optical microscope (TSOM) using information about the focal positions.
An image provided by the TSOM does not represent the object as it appears unlike a common photo (captured image). Although the images are abstract a little, a difference between the fine shapes of a measured target three-dimensional structure may be deduced based on a difference between the images.
Simulation research revealed that the TSOM is capable of measuring characteristics of 10 nanometers or less. The TSOM proposes the possibility of analyzing the shape of a fine three-dimensional structure.
However, to obtain optical images having many different focal positions with respect to a very small object is a task requiring a lot of time. A method which solves problems and which is substantially used for the illumination of a semiconductor apparatus has not yet been properly proposed.